The first integral is evaluated from 0 to 0.723 and the second is from 0.723 to 6.

Volume = (3.14/2)(0.3985 + 3.60654) = 6.29109 ft^2

Fill Calculation

6.29109(12^3) = 10871 cubic inches

10871 / 800 = 13.6 oz

It should be simple to replace thickness layer with another thickness(like 1 inch or 1.5 inches), or to make the sleeping bag longer or shorter than 6 feet(just change the linear slope so that it intersects that point).